Feature: Notifications — add real-time notifications via WebSockets

[Hexstar-labs]

Description:
Notifications are currently only available by polling GET /notifications. Real-time delivery improves UX.

Tasks:

• Install @nestjs/websockets and socket.io
• Create NotificationsGateway that authenticates connections via JWT
• Emit notification events to the user's socket room when a new notification is created
• Connect from the frontend using socket.io-client
• Show a notification bell in the Navbar with an unread count badge that updates in real time

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